What causes sheen variations in the topcoat?

Sheen variations and dull spots in high-gloss topcoats usually result from one of the following:

1. The topcoat has been applied unevenly.
2. Sandpaper used before application of the topcoat was too coarse.
3. The topcoat was applied in conditions which cause it to set up prematurely.  These include a substrate which is heated from the sun or where wind or ventilation is excessive.

You can avoid sheen variations by coating in an environment where the substrate is protected from the sun and wind.  Additionally, sanding the substrate to 320 will provide sufficient tooth and a scratch pattern which promotes a uniform final appearance. 

Dipping the brush or wetting out the roller with the topcoat charges (fills) each applicator with a given volume.  This volume of topcoat has an optimal area in which to be applied.  In the case of the brush, it will glide smoothly.  Once it begins to drag, the volume of topcoat within the brush is depleting. Avoid further brushing.  Recharge the brush, overlap into the previously applied material and dispense the topcoat. 

A freshly charged roller will apply topcoat smoothly and with little noise coming from the substrate.  Once the roller begins to make some surface noise, you know it's time to recharge with topcoat.  Again, slightly overlap with the previously applied material. Be consistent in charging and dispensing and you will get a much more uniform sheen over the surface.